It's one of those things that seems intuitive and shouldn't require explanation: Exercise makes you healthy.

But just how the two are linked has stumped scientists until a new study released Wednesday in Nature explained the role of a hormone, PGC1-alpha, that is produced in muscles during and after exercise. To sum it up:

“It seems clear that PGC1a stimulates many of the recognized health benefits of exercise,” said the lead author, Bruce Spiegelman, the Stanley J. Korsmeyer professor of cell biology and medicine at the Dana-Farber Cancer Institute and Harvard Medical School.

But read on, because it's not the only star hormone in this show.

How exercise affects fat

Basically, PGC1a starts a chain reaction whose end result affects fat. First, it causes a hormone that had never been identified and which they named irisin to arise in muscles. Then the irisin enters the bloodstream and hitches a ride to fat cells where, as the New York Times describes it, "it begins turning regular fat — especially deep, visceral fat clustered around organs — into brown fat."

Regular fat, brown fat -- what's the difference? Well, it turns out that some kinds of fat are better than others. According to the Times:

While white fat cells are essentially inert storehouses for fat, brown fat cells are metabolically active. They use oxygen and require energy. They burn calories.

It was once that adults lost brown fat after babyhood, but three years ago, studies showed that adults do have brown fat.

So, the bottom line is the exercise leads to the rise irisin which leads to a a decrease in white fat and an increase in brown fat.

The experiments

In one experiment, the researchers took white fat cells from mice and injected irisin into them and saw that the cells' genetic changes showed they were browning. The fat cells also appeared to be burning more energy.

Exercise also appeared to help prevent the development of diabetes. Mice who were fattened with fatty food and then injected with Fndc5 protein, which breaks apart into irisin, had better glucose tolerance and did not develop diabetes though their diet put them at higher risk for it.

In humans, researchers found that human volunteers who finished a controlled weeks-long jogging program had much higher levels of irisin in their cells after the program than before.

In the experiments, the irisin did not do much to help the mice lose weight. However, the mice did resist gaining weight even when they were eating a high-fat diet and their bodies also kept their blood sugar levels stable. The Times concludes, "So it would seem that exercise, through the actions of irisin, can render you healthy, if not svelte."